Note that the center frequency also should be the geometric mean of the passband cutoffs. The geometric mean of your passband frequencies is:
sqrt(100 * 3500) = 591Hz
Since this is different than the 800 Hz for the stopband frequencies calculated above, it means you can't get exactly all four of the stopband and passband cutoffs that you want. This is because bandpass filters are symmetric around the center frequency. Or another way of putting it is that you are starting with 4 constraints (passband high cutoff, passband low cutoff, stopband high cutoff, stopband low cutoff) , but only have three variables to play with (center frequency, passband bandwidth, stopband bandwidth).